The invention relates to a magnetic sensor employing magnetoelectric semiconductor transducer elements, and more particularly to a magnetic sensor which may be used for reading a magnetic pattern representing characters or symbols magnetically printed on a printed matter.
Sensors of both electromagnetic coil type and magnetoelectric transducer type are available for reading a magnetic pattern impressed or printed using a magnetic ink or other material containing magnetizable component. A sensor of the electromagnetic coil type is subject to a large variation in its output in response to a change in the gap between the printed surface and the sensor surface or when the speed of movement is changed, thus requiring a feed mechanism of high accuracy for the printed matter. In addition, such sensor requires that the printed matter be fed in a manner such that the printed surface is directed toward the sensor surface. Also, the either side of the printed matter must be determined, and the direction of feeding must be specified.
The sensor of magnetoelectric transducer type undergoes a small variation in the output when the gap between the printed surface and the sensor surface varies, allowing a simplified feed mechanism to be employed. It is unnecessary to determine either side of the printed matter or to specify the direction of feeding. Furthermore, the speed of movement of the printed matter need not be considered. However, magnetoelectric transducer elements formed of a semiconductor material respond to a leakage flux of a high frequency such as emanating from an electric motor or the like by producing noises which are interspersed with the output. A rapid change in the temperature of the elements results in a low frequency drift in the output response, thus preventing the achievement of a high S/N ratio. When subjected to a pressure or impact externally, the piezoelectric effect of the elements produces noises superimposed with the output pulse, which may cause a wrong operation of the sensor circuit.
Such magnetic sensors find an extensive application and hence are used under varying conditions. In particular, magnetoelectric transducer elements are susceptible to a rapid change in the temperature, the deposition of magnetic dusts, an external pressure or impact, and the influence of moisture, water or salt, which may cause a variation in the output voltage, the generation of noises and a decrease in the usefull life. To provide an arrangement which permits a magnetic sensor to be used under the conditions mentioned above, it is necessary to provide an enclosed structure. However, magnetoelectric transducer elements may not be contained in a housing having a thick wall in order to increase the magnetic field applied to the elements as high as possible. It is thus seen that it is desirable to dispose these elements adjacent to the outer surface of the housing. In view of this, the prior art include a housing formed with a through opening, in which the elements are disposed and sealed as by resin. However, the prior art housing has not been enclosed to a sufficient degree to withstand the described environmental influences, and has given rise to the above susceptibility with time.